1. Field of the Invention
The present invention relates generally to power supply topographies, and more specifically to power supply topographies for digital processing systems.
2. Background Information
It is well known that digital processing systems, such as personal computer systems, require a power supply in order to operate. In fact, it is common for the display device (e.g., video monitor) and central processing unit (CPU) of a “separated” personal computer system to be powered by separate power supplies. In a separated personal computer system, the display device is separate from the housing containing the CPU.
Even in “all-in-one” personal computer systems, in which the display device and CPU are housed in the same enclosure, a separate power supply is used for each of the display device and the CPU. In all-in-one computer systems, the power supply for the CPU is not necessarily located on the same printed circuit board that features the CPU which is located at main logic board (MLB). If the power supply is not located on the MLB along with the CPU, a group of cables is needed to connect such an off-board power supply to the MLB such that the necessary voltages can be supplied to components, such as the CPU, on the MLB. However, the cables used to connect the power supply to the MLB are not only relatively large and costly but they also require a noise rejecting filter to be placed at the junction between the cables and the MLB. Thus, using an off-board power supply and associated cables increases the cost and complexity of the system.
A commonly used power supply for a display device is a flyback converter. Flyback converters are generally used to generate relatively high voltages at low current. A basic circuit diagram of a flyback converter is shown in FIG. 1A. In a flyback converter, energy is stored in a transformer 105 during the power switch on time. During such time, the load current is supplied from an output filter capacitor 110. When the power transistor turns off, the energy stored in the power transformer 105 is transferred to the output load and replenishes the charge in the filter capacitor 110. A controller circuit 115 provides the necessary control and drive signals (base/gate drive signals) for switch 120. Diode 125 and capacitor 130 represent a conventional bridge rectification and capacitive filter needed to produce a high voltage DC supply for the flyback converter.
A commonly used power supply for a CPU is a forward converter, which is also referred to as a down converter. Forward converters are generally used to generate relatively low voltages at high current. A basic circuit diagram of a forward converter is shown in FIG. 1B. A forward converter is a switching power supply in which the energy is transferred from the input to the output during the on state of the primary switching device 150. A winding 155 of transformer 160 is phased so that energy will be transferred to the output and stored in the output choke 165 when the power transistor is on. When the power transistor turns off, the energy stored in the output choke is transfer to the output load. A controller circuit 180 provides the necessary control and drive signals (base/gate drive signals) for switch 150.
Generally, two independent power supplies are used to generate the power needed to drive a display and a CPU in an all-in-one computer system. The independent power supplies typically share only a few components related to the AC input and the AC electromagnetic compatibility (EMC) filter circuitry. FIG. 1C illustrates a basic block diagram of two independent power supplies 185 and 190 which provide power to a display 187 and a CPU (not shown) located on a printed circuit board, such as main logic board (MLB) 195. The independent power supply which drives the display 187 is a flyback converter 185, and the independent power supply which drives the CPU is a forward converter 190. Forward converter 190 is shown located apart from MLB 195. Thus, a group of cables 191 is needed to connect forward converter 190 to MLB 195 such that the necessary voltages can be supplied to components on MLB 195. A noise rejecting filter 193 is used at the junction between cables 191 and MLB 195.